I've been having a lot of fun working with MediaWiki and particularly the Semantic MediaWiki extensions. A few months ago a friend from when I worked at Dow Jones, Armistral, asked me for some input on how he could build this website he was working on. He wanted to create a site where people could work together to solve problems. Thus WeSolver was born. I strongly recommended that he use MediaWiki and he ran with it. The site is now live and he's done a nice job setting it all up. Check it out and if your so inspired see what you can do to help with a solution!